1. Field of the Invention
The present invention relates to a heat-resistant epoxy resin composition having excellent stability at normal temperature. More specifically, the invention relates to an epoxy resin composition that has a long pot life and, after it is cured, exhibits large resistance against deformation by heat (or a high glass transition point) and excellent resistance against degradation by heat.
2. Prior Art
So far, aromatic diamines and, particularly, diaminodiphenyl sulfones (hereinafter referred to as DDS) have been widely used as curing agents for liquid bisphenol A diglycidyl epoxy resins (hereinafter referred to as DGEBA epoxy resins) requiring, however, the reaction at a high temperature and for an extended period of time for being cured. As a cure promoting agent, therefore, a boron trifluoride amine complex compound is usually added resulting, however, in a decrease in the resistance against the water and electric properties to a large extent. Moreover, addition of the cure promoting agent affects the pot life.
There have further been known an anhydride of a tetrabasic acid and, particularly, an anhydrous pyromellitic acid as heat-resistant curing agents to substitute for DDS. They, however, have high melting points involving inconvenience from the standpoint of operation and are, hence, used being blended with other acid anhydrides. The epoxy resin using such acid anhydrides has a heat resistance which is a glass transition temperature (Tg) of as high as 202.degree. to 213.degree. C. but remains being readily to be used for only as short as about 30 minutes at 80.degree. to 90.degree. C.
It has been known already to use a dicarboxylic acid dihydrazide as a curing agent for epoxy resins, and the applicants are placing adipic dihydrazide, sebacic dihydrazide, isophthalic dihydrazide and dodecandicarboxylic dihydrazide in the market.
However, the conventional curing agents for epoxy resins were not still satisfactory in regard to stability at room temperature when they were blended in the epoxy resin, and resistance against the heat and resistance against the water when the epoxy resins were cured.
It has therefore been desired to develop a newly blended one part system heat-resistant epoxy resin composition by using an epoxy compound having two or more epoxy groups, the epoxy resin composition exhibiting excellent stability at room temperature, having property to undergo curing reaction at an increased rate contributing to shortening the time for molding, and the epoxy resin composition after cured exhibiting high Tg, excellent resistance against the water, resistance against degradation by heat and excellent properties in combination.
According to the present invention, there is provided a heat-resistant epoxy resin composition having excellent stability at room temperature comprising an epoxy compound having two or more epoxy groups and a curing agent which comprises a naphthalenedicarboxylic dihydrazide as a main component and, particularly, a one part system heat-resistant epoxy resin composition.
It is desired that the epoxy resin composition contains ceramic whisker as a filler from the standpoint of resistance against the heat and mechanical properties.
According to the present invention, furthermore, there is provided a curing agent for epoxy resins comprising a naphthalenedicarboxylic dihydrazide and, particularly, a 2,6-naphthalenedicarboxylic dihydrazide.
According to the present invention, the naphthalenedicarboxylic dihydrazide is used as a curing agent for epoxy resins. As will be described later by way of Examples, when added to the epoxy resin, the naphthalene dicarboxylic dihydrazide exhibits a pot life which is usually longer than 21 days at 40.degree. C. (which corresponds to 180 days at 25.degree. C.) yet exhibits excellent curing reactivity when heated. Besides, the resin after cured exhibits Tg which is not lower than 200.degree. C. and, often, reaching 260.degree. C., and further exhibits excellent resistance against degradation by heat and resistance against the water.
The naphthalenedicarboxylic dihydrazide used in the present invention has a chemical structure expressed by the following formula (1), ##STR1## and has a distinguished feature in that it contains a functional goup of dicarboxylic acid dihydrazide and has a naphthalene skeleton. Owing to this feature, there is obtained a cured resin which has an excellent pot life and quickly curing property that stem from the dicarboxylic acid hydrazide, and further has a high glass transition temperature and excellent resistance against degradation by heat and resistance against the water.
The naphthalenedicarboxylic dihydrazide used in the present invention is particularly useful as a curing agent for the epoxy resin that has an aromatic group in the main chain thereof. Based upon the combination of the two, there is obtained a cured resin having a high Tg and excellent resistance against degradation by heat and resistance against the water.
It is desired that the naphthalenedicarboxylic dihydrazide is used in an amount of from 10 to 70 parts by weight and, particularly, from 20 to 60 parts by weight per 100 parts by weight of the epoxy resin. When its amount is smaller than the above-mentioned range, an extended period of time is needed for the curing or the cured resin loses resistance against the heat. When the amount is larger than the above-mentioned range, on the other hand, the machinability of the blended resin and the pot life are deteriorated or mechanical properties of the resin are deteriorated.
The naphthalenedicarboxylic dihydrazide used in the present invention has a feature in that it offers an excellent pot life yet exhibiting compatibility to the epoxy resin and further works to increase viscosity of the blended resin. It is therefore desired to use, as epoxy resins, an epoxy compound (A) which remains solid to semi-solid at room temperature and a liquid epoxy compound (B) having an epoxy equivalent of not larger than 200; i.e., it is desired to use the epoxy compound (A) which is solid to semi-solid (viscous liquid) at normal temperature and the liquid epoxy compound (B) of a low viscosity at a weight ratio of from 10:90 to 90:10 and, particularly, from 30:70 to 70:30.